In the past few decades, the automotive industry has experienced a tremendous proliferation in both the number and types of vehicular functions and systems subject to computer control. As an example of one such system, a modern engine ignition system typically includes an ignition coil, a coil current switching device responsive to an ignition, or "drive", signal to energize the ignition coil, and some type of microprocessor-controlled circuitry to provide the drive signal to the ignition coil.
One consequence of such computer control is the possibility of a failure or fault condition associated therewith. If proper anticipatory measures are not taken, certain potential fault conditions can lead to undesirable, and often damaging, results. For example, one possible fault condition in an automotive ignition system may occur when the drive signal input to the ignition system remains on for an excessively long time period. Under such a fault condition, the coil current switching device may be damaged by high temperatures resulting from continuous, and prolonged, conduction of coil current.
To prevent such damage, it is desirable to limit the length of time that the ignition coil current is permitted to flow. However, since potentially damaging temperatures, due to an excessive duration drive signal, are a function of the battery voltage, less heat is generated as the battery voltage decreases. Thus, the maximum coil current flow time, or "lockup" time, should ideally increase as battery voltage decreases. What is therefore needed is an automotive ignition system lockup protection circuit wherein the lockup mime is inversely proportional to battery voltage. Such a lockup protection circuit should further operate consistently over a temperature range typically required in an automotive application, such as between approximately-40 degrees and 160 degrees Celsius (C).